1. Field of the Invention
The present invention relates to an optical encoder and, more particularly, to a small optical encoder in which manufacturing costs can be suppressed and detection accuracy is improved.
2. Related Background Art
FIG. 8 is a perspective view showing a conventional rotary encoder.
A rotary encoder 1 shown in FIG. 8 is employed, for example, in a tape recording/reproducing apparatus which does not use a capstan and is constructed so that the rotary shaft 2 of the rotary encoder is rotated with travel of a tape.
The rotary encoder 1 comprises a rotor 3 fixed to the rotary shaft 2 and a stator 4 partially facing the rotor 3. Slits 6 are patterned so as to circumferentially alternately provide passing regions and shielding regions of light like a bar code in the circumference of the rotor 3. In the stator 4, slits 5 are formed in a part of the fan shape. The slits 6 are patterned so as to overlap with the slits 5 formed in the stator 4 when the rotor 3 is rotated.
A light emitting diode (LED) or the like is used as a light emitting device 7 and a photo diode (PD) or the like is used as a photo sensing device 8. Light emitted from the light emitting device 7 passes through the slits 5 and 6 formed in the rotor 3 and the stator 4, detected by the photo sensing device 8, and converted to an electric signal (envelope signal). By processing the envelope signal by electric means (not shown), the rotational speed of the rotor, a phase deviation, and the like can be detected. Consequently, the speed at the time of travelling, wow and flutter, and the like of the tape recording/reproducing apparatus can be controlled.
In the conventional rotary encoder 1, since overlap information (envelope signal) of the patterns of the slits 5 and 6 is obtained by detecting only a part of the whole slits 6 formed in the rotor 3, the signal level is low and much noise is included, so that the accuracy is not high. In order to suppress the noise and to obtain a high output level, an expensive linear light source is necessary. Further, since a positional deviation between the light source and the center of the photo sensing device or a positional deviation between the slits 5 and 6 largely affects the performance of the encoder 1 itself, for example, the encoder 1 itself becomes the occurrence source of wow and flutter and the performance as an optical encoder becomes defective. On the other hand, when the performance is sought too much, a high degree of assembly technique is requested and a problem such that it cannot be easily assembled at the time of production occurs.
For instance, when it is assembled in a state where the centers of the rotary shaft 2, the rotor 3, and the slit patterns are deviated from each other by 5 .mu.m in the radial direction, it was confirmed that only an extremely weak envelope signal is outputted due to the deviation. Since the weak signal includes many noise signals, it exerts an adverse influence on subsequent electric processes. Consequently, whether a reliable envelope signal can be obtained or not depends on how accurately the two slit patterns can be set and the concentric precision (concentricity) between the rotor in which the two slits are formed and the rotary shaft can be obtained.